Optical fibers are commonly used in sensors, typically to measure some form of energy. Changes in the energy can affect changes in the fiber disposition, location, and configuration that manifest themselves for example as microbends. Light being transmitted through the optical fiber experiences changes in amplitude or phase due to the effect of microbends or other changes in fiber length.
Strain sensors have been developed to sense localized strain, that is, the strain present at discrete locations along the optical fiber. This localized strain corresponds to the energy present at a particular location along the fiber. One such strain sensor is described in U.S. Pat. No. 4,459,477 to Asawa et al. (hereinafter the Asawa et al. patent), where optical pulses are transmitted into the fiber. Microbend transducers dispersed along the exterior of the optical fiber cause changes in the amplitude in the transmitted optical pulses. The change in amplitude experienced by the optical pulses corresponds to the energy present with respect to a microbend transducer in a localized portion of the fiber. However, significant problems with the use of microbend transducers and optical fibers are the large power losses, the transient effects and the complex apparatus required to measure strain.
Another patent that relies on discontinuities to measure strain is U.S. Pat. No. 4,653,916 to Henning et al. (hereinafter the Henning et al. patent). Henning et al. and Asawa et al. patents rely on discontinuities produced by external means such as transducers. This relatively complex, burdensome and obtrusive system requires extensive external apparatus to measure localized strain. Because the discontinuities are external, strain in various locations throughout the cross-section of the structure cannot be measured. In the invention disclosed herein, the system of optical fibers and splices can be located throughout the cross-section of a structure to yield an array of strain and temperature measurements.
A feature of the invention avoids the need to rely on microbends and rather uses measurement techniques that correspond to changes in a gap between the fibers in a particular slice. This change corresponds to a change in pulse amplitude which can be utilized to measure temperature or strain. A sensor of the invention is comprised of optical waveguides used with a unique splice configuration. Two fibers of a low coefficient of thermal expansions, such as sapphire, are held within a metal tube by high temperature epoxy. The ends of the fibers are exposed within the tubes in spaced relationship to form an air gap therebetween. The air gap opens or closes depending on the load imposed by changes in temperature or strain and the amount of light transmitted varies accordingly. In this manner various locations along a particular structure can be measured for temperature and strain.
The above has been a brief description of the deficiencies of the prior art and advantages of the invention. Other advantages will be apparent from the detailed description of the preferred embodiment which follows.